1. Field of the Invention
The present disclosure generally relates to devices for and methods of protecting an actuator against overload conditions; and more particularly, to a device for and method of protecting an actuator against both overloading and overheating that utilize superelastic (also known as “pseudoelastic”) shape memory alloy actuation.
2. Discussion of Prior Art
Active material actuators, such as shape memory alloy wires, are generally protected from mechanical overload by the use of mechanical springs or electrical control schemes to avoid damage to the actuators when the output load exceeds a recommended limit. Both of these measures, however, present various concerns in the art. For example, mechanical springs needed for overload protection tend to be bulky because of the conflicting requirements of high force threshold for the overload function necessary to enable normal operation, and low stiffness to restrict the maximum stress experienced by the actuator when the overload protection system is activated. The electrical/control schemes are more versatile, but they increase system cost.
More particularly, conventional solutions typically employ pre-loaded linear springs. In these systems, the pre-loads in the springs are typically set when the springs are manufactured or individually imposed through mechanical constraints, such as pre-load screws. The resulting protection effects high stiffness until the force in the actuator exceeds the pre-load force level; beyond this point, the overload protection system exhibits a stiffness corresponding to the native stiffness of the linear spring. This leads naturally to conflicting requirements on the design of the overload protection system. A high pre-load force requires either an overload spring with a high native stiffness or an overload stiffness spring with low native stiffness that is pre-loaded through a significant part of its useful deflection range to achieve the high preload force. The former approach leads to a compact design for the overload protection system but results in high maximum force levels in the actuator during a mechanical overload event; while the latter approach results in a much lower maximum force level in the actuator during a mechanical overload event, and leads to an unwieldy overload protection system due to the large undeflected size of the overload protection spring.